Studies on the Radioactive Contamination of Fruit Trees by Fission Products (3) Distribution of the Replaceable 137Cs in the Soil of a Citrus Orchard

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Since 1961 through 1971, the replaceable ^<137>Cs content in the same surface soil of the same area under clean, mulch and sod systems of soil management at a citrus orchard with that described in the previous paper1) was determined in order to compare its annual change with that of replaceable ^<90>Sr. The relation of humus content in surface soils classified as clean <sod < mulch closely related to half-time for an exponential decrease of replaceable ^<137>Cs from 1963 through 1971 and with distribution coefficient1) for the downward distribution of replaceable ^<137>Cs in surface soil. Both half-times of approximately 8.5, 45.2 and 30.7 years and distribution coefficient of 0.094-0.111, 0.142-0.165 and 0.129-0.140 for clean, mulch and sod, respectively, are larger than those of ^<90>Sr. The ratios of replaceable ^<137>Cs to ^<90>Sr of 0.92-1.13 in surface soil found for the year from 1961 through 1967 followed by a slight increase afterwards and reached 1.24-1.36 in 1971. It was found that ^<137>Cs is fixed much more strongly than ^<90>Sr in the surface soil of a citrus orchard.
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